This invention relates to an electronic article surveillance system and, in particular, to a system in which a clear line of demarcation is to be maintained between a surveillance zone and adjacent areas.
U.S. Pat. No. 4,686,513, issued Aug. 11, 1987, for "Electronic Surveillance Using Self-Powered Article Attached Tags", and assigned to the same assignee as the present application, describes a surveillance system in which interrogation signals are transmitted by a surveillance transmitter into a surveillance zone. A surveillance receiver is then provided to receive signals transmitted from self powered tags situated in the zone. These tags each comprise a tag receiver for receiving and decoding the interrogation signals transmitted by the surveillance transmitter and a tag transmitter responsive to the receipt of a valid interrogation signal for transmitting an alarm signal to the surveillance receiver.
As described in the '513 patent, it is desirable to have clearly defined lines of demarcation between the surveillance zone and certain adjacent areas (e.g. article display areas), where articles having attached tags may be present prior to checkout and removal from the premises. Failure to have such lines of demarcation can result in tags, located in these adjacent areas, inadvertently responding to interrogation signals and erroneously initiating alarm transmissions.
In the '513 patent, a clear line of demarcation is maintained between the surveillance zone and desired adjacent areas by transmitting, in these areas, a further digital signal which is synchronous with and complementary to the interrogation signal. As a result, the interrogation signal and the further signal combine in these areas to form a composite signal which has no meaningful signal content when decoded by a tag's receiver. Therefore, tags subject to the composite signal do not generate a response, as they would if only an interrogation signal were present.
In the above-described system, the transmitters for the interrogation signal and further signal are synchronized to prevent the further signal from being transmitted during a time period in which an alarm transmission is expected from a tag. Failure to properly synchronize the transmitters could result in the surveillance receiver being interfered with or jammed by the further signal and, therefore, prevented from properly receiving such an alarm transmission. The need to provide for synchronization, however, results in an increased system cost and complexity.
It is, therefore, an object of the present invention to provide an improved electronic article surveillance system.
It is a further object of the present invention to provide an electronic article surveillance system in which a clear line of demarcation is maintained between a surveillance zone and adjacent areas without adversely effecting the surveillance system receiver.